1. Field of the Invention
The present invention relates to a solid state laser apparatus for use as a light source in such fields as optical recording, communication and measurement.
2. Description of Related Art
FIG. 23(a) is a front view showing a structure of conventional solid state laser apparatus. The solid state laser apparatus includes a light source emitting a pumping radiation 66 for exciting a lasant material 62, a lens system 61 for focusing of the pumping radiation 66 and an optical resonator 65 including the lasant material 62, a non-linear optical element 63 and a concave mirror 64, which are disposed on an optical axis 68. Here, such example is described that the lasant material 62 is formed of Nd:YVO.sub.4 doped with about 1% of Nd, which is a positive uniaxial crystal. The non-linear optical element 63 is formed of KTiOPO.sub.4 which is a positive biaxial crystal. Finally, a semiconductor laser of 809 nm in oscillatory wavelength is employed as the light source 60.
When the pumping radiation 66 from the light source 60 is focused by the lens system 61, and enters the lasant material 62, an inverted distribution is formed in the lasant material 62. Thus, in the case of Nd:YVO.sub.4 crystal, a light amplification of 1064 nm in wavelength is allowed. A surface 62a of lasant material 62 is coated with a coating which has a high transmissivity at a wavelength of 809 nm that corresponds to the pumping radiation 66 and a high reflectance at a wavelength of 1064 nm. A surface 64a of the concave mirror 64 is coated with a coating which has a high transmissivity at a wavelength of 532 nm and a high reflectance at a wavelength of 1064 nm. Thus, an optical resonator at a wavelength of 1064 nm is formed of the two surfaces 62a and 64a.
As a laser beam of 1064 nm in wavelength oscillated by a light amplifying effect of the laser material 62 passes through the non-linear optical element 63, it is converted to 532 nm in wavelength, which corresponds to a second harmonic, and an output beam 67 is obtained. Here, surfaces 63a and 63b of the non-linear optical element 63 are coated with coatings which have a high transmissivity at wavelengths of 1064 and 532 nm. A surface 62b of the lasant material 62 is coated with a coating which has a high transmissivity at a wavelength of 1064 nm.
FIG. 23 (b) is a partial perspective view showing an arrangement of polarization axes of the lasant material 62 and the non-linear optical element 63 shown in FIG. 23(a). Nd:YVO.sub.4 forming the lasant material 62 is a positive uniaxial crystal. A principal index of refraction nc is higher than two other principal indices of refraction na. Further, c-axis that is an oscillation facilitating axis is directed vertically upwardly to the optical axis 68. Finally, an a-axis of the principal indices of refraction na is positioned so as to coincide with the optical axis 68.
On the other hand, KTiOPO.sub.4 forming the non-linear optical element 63 is a positive biaxial crystal, and is cut, when x-, y- and z-axes are selected so that principal indices of refraction of an index ellipsoid are nx&lt;ny&lt;nz, in parallel with a z-y' plane that is inclined at angles .theta.=90.degree. between the z- and y'-axes and .phi.=21.3.degree. between the y- and y'-axes (J. Q. Yao and Theodore S. Fahlen, J. Appl. Phys. vol. 55(1), 65 (1984)). Therefore, the z- and y'-axes come to be polarization axes, x'-axis coincides with the optical axis 68, and the y'-axis is arranged so as to be inclined at an angle .alpha.=45.degree. to the c-axis of lasant material 62, as viewed from the direction of optical axis 68. In addition, because the non-linear optical element 63 is controlled by temperature adjustment or the like, so that a phase difference by one-way retardation at a specified wavelength of longitudinal mode comes to be m.pi. (where m is an integer), polarization in the lasant material 62 is maintained in a linear polarization parallel with the c-axis, the oscillation facilitating axis.
In such a solid state laser apparatus, the output beam 67 is emitted to the outside as a coherent light of 532 nm in wavelength through the concave mirror 64, because a laser beam of 1064 nm in wavelength enclosed within the optical resonator is harmonically converted in the non-linear optical element 63 in so-called type II phase matching condition, where a second harmonic is produced by existence of fundamental waves linearly polarized in the z- and y'-axes.
As another example, a laser with Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 employed as a lasant material is disclosed in Japanese Laid-Open Patents (KOKAI) No. 91-174786 and 91-88380, wherein a beam walk-off is caused in the lasant material, and a laser oscillation is initiated along the oscillation facilitating axis.
In a conventional solid state laser apparatus, however, as adjacent longitudinal modes in the optical resonator 65 are less distinctive by a gain difference between them, and even in the case that a laser oscillation is caused in a longitudinal mode with a maximum gain, another laser oscillation is also allowed in an adjacent longitudinal mode with a non-maximum gain due to a harmonic conversion loss. It is a problem that a so-called mode-hop phenomenon, that is an oscillation mode to be changed in time course between longitudinal modes for possible oscillation, is caused, and the output beam 67 varies in intensity. Such mode-hop phenomenon is caused also in a general optical resonator including two or more double refraction optical elements.
In order to prevent such mode-hop phenomenon, it is required to match a frequency of the longitudinal mode determined by a length of the optical resonator with a gain peak frequency of the lasant material 62 by finely adjusting a positioning angle of the non-linear optical element 63, concave mirror 64 and the like in a order of 0.1.degree.. For example, as such adjusting operation is very difficult, it is another problem that a production yield is reduced. Those with an oscillation facilitating axis in lasant material have been described above. On the contrary, in the case that a lasant material is isotropic, because a gain does not depend on polarization, longitudinal mode is not completely discriminated. Therefore, although such polarizing element as Brewster a plate may be separately inserted in an optical resonator, it is a problem that an apparatus is thereby increased in size.